Electronic audio switch



April 20, 1948. w, D, HOUGHTQN 2,440,049

ELECTRONIC AUDIO SWITCH Filed June 30, 1945 ATTORNEY.'

Patented Apr. 2Q, 1948 UNITI-:fo oFeFicr-z signor toRadio-fGorporation fof America; a corporation iofSfDelaware is utilizedY to 95Claims.

This invention relates Vto an electronic'switching circuit.and'particularlyto av method of and rent (for example, :20 cyclespper 'sec.ond)fwhich actuatejan alarm 'and thus4 attract the attention'of-the loperator at the `receiving end of-the system.

Briefly, the inventiongcomprises a vacuumtube which is'norma'lly 4biased to the anode current cut-off condition, and ;to"w1:1ichbotl'1v the audio modulation and the ringing-voltage' are applied. The application of the ringing voltage causes the vacuum tube to conduct-and produce an eiective short-circuit (low impedance path) across a resistor across which the audiornodulationis-applied .through acondenser. This lov/.impedance pathreduces the effective a Vplitude of the modulation waves across the resistor to ai lowvalue.

Aieature of the invention-lies-iny the .circuit scheme which converts the low frequency ringing voltage toa directcurrent voltagefwhichis used, in one application offtheinventionto a pulse `signalling system, to advance .the 4time at which a pulse is `transmitted and at :the same time -reduces the amplitude of v-the-audio Avoltage applied to the system.

A more detaileddescripticnof the-invention follows, accompanied by drawings, wherein-the same parts throughontthe figures lare represent ed by the same reference characters.

.In the drawings:

Fig. 1 is a circuit-diagramgiven toiillustrate the principles of ythe present invention;

\ Fig. 2 illustrates `ageneral n.application-.of Yth present invention; l.and

Fig. 3 illustrates t-he invention as `applied 1:to a pulse. generatorv modulator circuit.

Fig. 1 illustrates-,the basic circuit ofV the-invention and is givenrfor `thefpur-posc ofc1eaily,;setting forth theprinciples involved. -This figure illustrates a triode vacuumtube VLhav-.ingan anode A to which-a. positive: potentiaLissupplied from a unidirectional .current source v +B.. a grid G, and a cathodeK. Inthelcathode*circuitithere is provided a rather Ahigh valueof :resistorfR/i connected between the cathode andogrou-ud. Connected between the grid andground there is provided a resistor R2 in series with a .battery,-E, which is so poled .as to supplyeJ negative potential to the grid to thereby bias Athe Vvacuum tube 'Vi normally .below enti-off. A. switch .S isfshown shunt 'ori short-circuit lower value.

grid and; ground `so (as 'to resistor R2 and batteryE :when'the' switch is in 'closed position. A suitable source of aufdiofrequencyinputwaves,for example speechmodulationvi/aves, is shown connected totermin'als l .and"`2. Terminal'Z isconnected to` ground while terminal I connected through condenserY Cltothe cathode K ofthe tube. 'The output-terminals .are labeled 3 and l4 andj these areconnected respectively tofthe cathodeK and Ltogrounid. 'ResistorRl has a Vvalue sumciently high sothat theau'dio response in output terminals L3 and `4Jis satisfactory.

.When switch VS is closed,'the negativefbias on the grid G ls removed and .the normally Acut-.01T or non-conductive tube .VI wi1l.conduct as a result of which the-impedanceacross resistor .-RI becomes low `for ithealternating current input wavesdeveloped across resistorlRl andemanatconnected" between 'the ingfrom'terminals I and.2. Whentube Vl .con- .ducts-most ofthe alternating lcurrent Vaudio input voltage iscausedto be developed across condenser CI, resulting in a reductioniin lthe amplitude of theaudio voutput voltage appearing on output-terminals' and 4.

AIn" eiect,.whenftube Vl conducts, vthe .effective impedanceacross resistorRl is reduced from the relatively high value oftheV resistor RI to a much 'Condenser -CI has normally a ,low impedance compared'tol RI for Yaudio voltages but.ltsimpedance.-isjrather high compared to the impedance nshunte'd .across resistor (Rl for Vthese sameaudio .voltages during thetime tube'l/Tiv conducts, as a result of which mostofthe audio voltage .is developed across the ,condenser CI when tube' Vlis nconducting. The low. 4impedance across resistorjRl acts .as an ,effective short :circuit lor lowimpedance path to ground for theaudio `voltage developed across condenser `CI `.and prevents itseiective application to output terminal .3.

"The effective impedance shunted acrossresistor` RI whenthe tube conducts .is givenfby the formula V -1+R1Gm whereGmviszthe transconductance of the'vacuum tube. i lIf R Lis. `for examplev .1f megohm,-and .the Gm of -the- :vacuum `tube used is 2000' micromhos, 4the impedance 4oiered -by rtube "VI' to vthe V.audio input wavesiis approximately 500woluns with :the switch S closed. "Hence,v when resistor fR! `appears `as f500-ohms,-most1ofthe voltage drop willbe across vcondens'erl C-I. v

'Figc Zaillustratesone embodiment Vof the inven- 4tionfwherein: a low frequency ringingvoltage (for example 20,cycles per'second') is rectified andthe resultant JD. Gaapplied to -thegrid of tube V I"for effectively4 :causing a reduction -in `amplitude 4of theo utput-'voltage` The circutgoi-Fig. 2i s..es,sen-

tially the same as that of Fig. 1, with the exception that the triode tube VI is normally biased below the anode current cut-off condition and is caused to conduct by rectifying the audio ringing voltage applied to terminals and 6. The ringing voltage applied to terminals 5 and 6 is passed through condensers C3 and C`4 to a diode rectifier V2 having in circuit therewith a suitable filter network C2, R2. This ringing voltage is rectied by diode V2, causing a D. C. voltage to be developed across resistor R3 of such nature that a voltage of positive polarity is coupled to grid G via resistor R2. The constants of the lter C2, R2 are such that the D. C. voltage developed across condenser C2 and applied to grid G is constant for the duration of the applied ringing. This positive voltage on the grid G is of suiicient value to overcome the cut-off bias on the tube VI and cause the tube VI to conduct, thereby causing l voltage to be developed across resistor RI which is positive with respect to ground at the cathode K. The result of tube VI conducting is to reduce the eiective impedance across resistor RI and consequently reduce the audio modulation fed to output terminals 3 and 4 while at the same time producing a positive voltage relative to ground at terminal 3. This last positive voltage is approximately equal to the positive voltage applied to the grid G. The low impedance across resistor RI acts as an effective short circuit for the audio voltage developed across this resistor, via CI.

In one embodiment of the invention in Fig. 2 the tube VI was an RCA 6SL7 type tube, the resistor RI had a value of one-half megohm, the condenser CI had a value of .01uf, the condenser C2 had a value of .111,1, the resistor R2 was 270,000 ohms, the resistor R3 Was 270,000 ohms and resistor R4 was 270,000 ohms.

Fig. 3 illustrates how the electronic audio switch can be applied to a phase modulator and pulse generator shown to the left of the vertical dash line. Such a phase modulator and pulse generator is described by way of example in my copending application Serial No. 617,634 filed September 20, 1945, describing a pulse signaling system. The electronic audio switch of the invention comprising vacuum tube VI and its associated circuit elements, operates in a manner described above in connection with Fig. 2.

The phase modulator and pulse generator comprises a triode vacuum tube V3 to which is applied (via terminal 1) periodically recurring saw-tooth voltage waves. Tube V3 is normally biased to cut-off and has its cathode connected to the anode of a modulator tube V4. Tube V4 acts as a variable resistor.

In effect, tube V3 is a pulse generator which is so biased as to start to conduct at about the middle of the applied saw-tooth voltage wave in the absence of audio modulation input from terminals I, 2. The modulation on terminals I and 2 is applied to tube V3 by means of tube V4 which serves to vary the bias on tube V3 and hence to vary' the critical value or time at which tube V3 starts to conduct 'over the slope of the saw-tooth wave impressed on the terminal 1. Transformer TI is a differentiating transformer which provides a pulse output at terminal 8 of sharp rise and decay when a rapid current change occurs in tube V3. The normal bias on tube V4 is set by the tap on its cathode resistor R5, and hence the variation in grid voltage of tube V4 changes the applied anode resistance of this tube and hence the cathode bias of tube V3. Any appreciable ow of current through tube V3 must be through tube 4 V4 and R2 has a very high value of resistance.

The maximum advancedposition in time of tube V3 due to modulating voltage is determined by the minimum anode resistance of tube V4 (maximum conduction position) which occurs when positive voltage is applied to the grid of tube V4. When ringing is applied to terminals 5 an-d 6, the tube VI will conduct and apply a positive D. C. voltage on the grid of tube V4 for the duration of the ringing. This positive voltage on the grid of tube V4 will cause this tube to assume its maximum conduction position, thus advancing the conducting time of pulse generator tube V3 relative to the saw-tooth voltage wave, as a result of which the pulse at terminal 8 in the output of the generator is advanced in time compared to the position which this output pulse would normally occupy when tube VI is cut oii. During this applied ringing, the audio applied at terminals I and 2 is reduced due to the previously described action of tube VI.

The invention is not limited to the use of a condenser CI since this particular circuit element can be any impedance, such as a capacitor (as shown), an inductor or a resistor, and the operation would be substantially the same. By way of example, let it be assumed that CI is a resistor equal in value to RI, i. e. 100,000 ohms and that tube VI has a Gm of 200 micromhos. Then when tube VI is cut off, the voltage at terminals 3 and 4 would be one-half or 6 db. less than the voltage at terminals I and 2. When tube VI is conducting, the shunted impedance is approximately ohms as obtained from the formula The voltage at 3 and 4 in this case is .005 times the voltage at the input or 46 db. less than the level at I and 2. Hence a'reduction of 40db. can be obtained.

The term ground used in the appended claims is deemed to include any point or surface of zero potential for D. C. or alternating current.

What is claimed is:

1. An electronic switch comprising a vacuum tube having an anode, a cathode and a control electrode, a source of unidirectional potential supplying positivepotential to said anode relative to the cathode, a resistor connecting said cathode to ground, means normally biasing said tube to the anode current cut-oit condition, said means including a resistor in series with a source oi unidirectional potential connected between said control electrode and ground, an output terminal for said system comprising a connection to sai-d cathode, a condenser having one side connected to said cathode, the other side of said condenser being adapted to be connected to a source of'audio voltage, a rectier in circuit with said grid for rectifying an alternating current voltage adapted to control said switch, to thereby supply said grid with a positive potential of suiiicient magnitude to overcome said bias and cause said tube to conduct, and a iilter coupled to said grid and rectiiier and having such constants as to assure a substantially constant rectiiied voltage for said grid for the duration of the applied alternating current control voltage.

2. An electronic switch comprising a vacuum tube normally biased beyond cut-off, said tube including a cathode and a grid, an output lead connected to said cathode, a pair of connections for supplying an alternating current voltage whose effective value in said output lead is to be controlled, a relatively high value resistor connecting one of said connections to said cathode, a condenser connecting the other of said connections to said cathode, and means for controlling said tube to reduce the output thereof in accordance with another alternating current voltage, said means including a rectier in circuit with said grid for rectifying said last alternating current voltage and for supplying said grid with rectiiied voltage of positive polarity and suflicient magnitude to overcome said bias and cause said tube to conduct.

3. A n electronic switch comprising a vacuum tube normally biased beyond cut-oli, said tube including a cathode and a grid, an output lead connected to said cathode, a pair of connections for supplying an alternating current voltage whose effective value in said output lead is to be controlled, a relatively high value resistor connecting one of said connections to said cathode, a condenser connecting the other of said connections to said cathode, an-d means for controlling said tube to reduce the output thereof in accordance with another alternating current voltage, said means including a rectier in circuit with said grid for rectifying said last alternating current voltage and for supplying said grid with rectied voltage of positive polarity and suilicient magnitude to overcome said bias and cause said tube to conduct, and a lter in circuit with said grid providing a substantially constant rectified voltage to said grid for the duration of the controlling alternating current voltage.

4. An electronic switch comprising a vacuum tube normally biased beyond cut-off, said tube including an anode, a cathode and a grid, means for supplying said anode with positive polarizing potential relative to the cathode, an output lead connected to said cathode, a pair of connections for supplying an alternating current voltage whose effective value in said output lead is to be controlled, a resistor `of a relatively high value connected at one end to said cathode, the other end of said resistor being connected to one connection, the other of said connections including a serially arranged condenser also connected to said cathode, the impedance of the condenser being low compared to the impedance of said resistor for said alternati-ng current voltage, and means for supplying said grid wth a positive potential of suiicient magnitude to overcome said bias and cause said tube to conduct, to thereby reduce the alternating current output voltage in said output lead.

5. An electronic switch comprising a vacuum tube normally biased for a predetermined limiting conduction condition, said tube including an anode, a cathode and a grid, means for supplying said anode with positive polarizing potential relative to the cathode, an output lead connected to said cathode, a pair of connections for supplying an alternating current voltage whose elective value in said output lead is to be controlled, a resistor of a relatively high value connected at one end to said cathode, the other end of said resistor being connected to one of said connections, the other of said connections including a serially arranged condenser also connected to said cathode, and means for supplying said grid with a potential of such polarity and magnitude as to overcome said bias and change the condition of said tube from said predetermined limiting conduction condition to another conduction condition.

6. An electronic switch in accordance with claim 4, including a rectier and a filter circuit coupled to said grid for enabling the control of said tube from another alternating current voltage.

7. An electronic switch comprising a vacuum tube normally biased beyond cut-off, said tube including an anode, a cathode and a grid, means for supplying said anode with positive polarizing potential relative to the cathode, an output lead connected to said cathode, a pair of connections for supplying an alternating current voltage whose elective value in said output lead is to be controlled, a resistor of a relatively high value connected at .one end to said cathode, the other end of said resistor being connected to one connection, the other of said connections including a serially arranged impedance element also connectedto said cathode, the impedance of the impedance element being low compared to the impedance of said resistor for said alternating current voltage, and means for supplying said grid with a positive potential of suiicient magnitude to overcome said bias and cause said tube to conduct, to thereby reduce the alternating current output voltage in said output lead.

8. An electronic switch comprising a vacuum tube normally biased beyond cut-off, said tube including a cathode and a grid, an output lead connected to said cathode, a pair of connections for supplying an alternating current voltage whose elective value in said output lead is to be controlled, a relatively high value resistor connecting one of said connections to said cathode, an impedance element connecting the other of said connections to said cathode, and means for controlling said tube to reduce the output thereof in accordance with another alternating current voltage, said means including a rectier in circuit with said grid for rectifying said last alternating current voltage and for supplying said grid with rectified voltage of positive polarity and suilicient magnitude to overcome said bias and cause said tube to conduct.

9. An electronic switch comprising a vacuum tube normally biased beyond cut-off, said tube including a cathode and a grid, an output lead connected to said cathode, a pair of connections for supplying an audio voltage whose effective value in said output lead is to be controlled, a resistor connecting one of said connections to said cathode, an impedance element connecting the other of said connections to said cathode, and means for controlling said tube to reduce the output thereof in accordance with an alternating current voltage, said means including a rectier in circuit with said grid for rectifying said last alternating current voltage and for supplying said grid with rectied voltage of positive polarity and suicient magnitude to overcome said bias and cause said tube to conduct.

WILLIAM D. HOUGI-ITON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number 

